This invention relates generally to gas turbine combustors and more particularly to improvements in gas turbine combustors for reducing combustion-induced instabilities.
In power plant design, reducing emissions of harmful gases such as NO.sub.x into the atmosphere is of prime concern. Low NO.sub.x combustors employing lean premixed combustion are being developed to address this problem. One such combustor comprises a plurality of burners attached to a single combustion chamber. Each burner includes a flow tube with a centrally-disposed fuel nozzle comprising a cylindrical hub which supports fuel injectors and an air swirler and has a flat face on its downstream end. In addition to a premix-injection stage for low NO.sub.x operation, each fuel nozzle can include a diffusion-injection stage for start-up and emergency operations and a liquid fuel-injection stage for liquid fuel operation. Diffusion gas fuel and liquid fuel are typically injected via orifices located on the flat end face of the fuel nozzle.
During low NO.sub.x (premix) operation, fuel is injected through the fuel injectors and mixes with the swirling air in the flow tube. The diffusion and liquid fuel circuits are typically purged with air during premix operation to keep flame gases out of the passages. The combustion flame is stabilized by bluff-body recirculation behind the fuel nozzle and swirl breakdown, if swirl is present. With premixed systems, strong pressure oscillations are typically produced as a result of combustion instabilities. The combustion instabilities are believed to be related to the shedding of spanwise vortices from the bluff end of the fuel nozzle.
These pressure oscillations can severely limit the operation of the device and in some cases can even cause physical damage to combustor hardware. Furthermore, the flow of purge air through the diffusion and liquid fuel circuits is injected directly into the recirculation zone. This direct injection reduces the local temperature and strength of the recirculation, producing an adverse effect on flame stability.
Accordingly, there is a need for a low NO.sub.x combustor which reduces pressure oscillations and avoids the adverse effects of injecting purge air directly into the recirculation zone.